WebGL: Validate texture copying feedback loops. This adds basic validation for catching CopyTex{Sub}Image calls whose source and destination textures overlap. It does not yet implement full support for ES3 types (3D textures, array textures). BUG=angleproject:1685 Change-Id: I83e7b1998df5575057fed8f99f7ee9970fb38df0 Reviewed-on: https://chromium-review.googlesource.com/425491 Commit-Queue: Jamie Madill <jmadill@chromium.org> Reviewed-by: Corentin Wallez <cwallez@chromium.org> Reviewed-by: Geoff Lang <geofflang@chromium.org>